Current aircraft, which operate at speeds close to the speed of sound, need to overcome a series of problems associated with flying at that speed. Hence, as the aircraft approaches the speed of sound, a rapid rise occurs in aerodynamic resistance over the aircraft, due to the effect of the compressibility of the air, at the same time as a loss of lift of the aircraft occurs along with a change in its pitching moment, which can affect its stability and controllability.
The critical speed at which the aforementioned compressibility effects occur increases, at the same time as the indicated adverse effects are minimised, if the aircraft's lift and stabilising surfaces are designed with a high sweep angle. The sweep of the aircraft's aerofoil and stabilising surfaces, or the inclination of these surfaces in the direction of flight is, therefore, a design characteristic for aircraft which fly at speeds close to the speed of sound and is motivated by aerodynamic considerations.
The aerodynamic advantage of the sweep lies in the fact that the adverse effects of compressibility, produced by the overspeed of the current over the aerodynamic profile, which increase with the relative thickness of this profile, are related to the component essentially perpendicular to the line of 50% of the chord of the surface of the air current incident on the aircraft. Therefore, for a given airspeed, an aerodynamic surface profile with a given sweep will be subject to compressibility effects equivalent to those of a profile with no sweep but with a profile of a thickness equal to the cosine of the sweep angle. A greater relative profile weight will result in lower structural weight of the aerofoil as the internal loads produced by the aerodynamic loads are reduced due to the increased girder of the structure. However, in high-speed flight, characteristic of modern large commercial aircraft, aerodynamic profiles with high relative thicknesses lead to the adverse effects from air compressibility, which can come to exhibit themselves as shock waves on these profiles, with the associated increase in aerodynamic resistance and other adverse phenomena for flight. Therefore, the sweep of the aircraft's aerofoil and stabilising surfaces serves to achieve a design compromise between the structural weight of these surfaces and their acceptable behaviour in flight at speeds close to the speed of sound.
The first aeroplane built for high-speed flight with a significant sweep angle was the Junkers 287 in 1945. Among other unusual features of this aeroplane, it can be highlighted that the sweep angle of the wings is negative, i.e. the wingtips are forward in the direction of flight with respect to the wing root, or connection of the wings to the fuselage. Except for very rare exceptions, such as the MBB/HFB 320, the Grumman X-29 and the Sukhoi 47, all with wings with negative sweep, the great majority of high-speed aeroplanes have been built with wings with positive sweep.
The effects of the sweep angle on the aerofoils are beneficial for high-speed flight, as described above. However, in low-speed flight phases, particularly during takeoff and initial ascent, as well as final approach and landing, aerofoils with high sweep angles have a greater tendency to lift stall at lower angles of attack than for aerofoils with no sweep. This is particularly a problem in the case of the wings, which require complex devices for high lift, such as the so-called flaps, to improve aerodynamic characteristics at low speed. In the case of the stabilisers, the need to incorporate sweep for flight at high speed results in lower efficiencies at low speed, which need to be compensated by increasing the area, and hence weight, of the aforementioned stabilisers. Additionally, the sweep in the stabilisers has the effect of reducing the lift force gradient with the angle of attack, which reduces the effectiveness as a stabilising surface on producing less force for a given wash angle.
Aircraft designs are known with aerofoils (wings) with high positive sweeps (of up to 60°) and with relative thicknesses not greater than 0.06:1. These designs are appropriate for aircraft flight at speeds close to and above the speed of sound, but they raise problems in flight at low speed required, for example, for takeoff and landing of the aircraft. Hence, aircraft designed with high sweep angles for high-speed flight, close to the speed of sound, would need to land or take off at much higher speeds than the same aircraft, but designed with conventional aerofoils (wings), with no sweep, or they would need to have aerofoils (wings) with a very high relative thickness, with the consequent increase in aircraft weight and resistance.
Due to this, as has been mentioned, the characteristics required for aircraft aerofoil (wing) profiles for high speed are contrary to those required for low speeds. Aircraft designs are known for which the sweep of the aerofoils is variable depending on the speed of the aforementioned aircraft, such as is the case, for example, of documents U.S. Pat. No. 4,569,493 and U.S. Pat. No. 5,984,231.
A small number of aeroplanes have made use of the concept of variable wing sweep, generally implemented such that each half-wing can pivot independently, but simultaneously, with respect to the fuselage, varying its sweep angle according to flight condition. This concept has been used in military aeroplanes such as the Grumman F-14, General-Dynamics F-111, Panavia Tornado, Mig-23, Mig-27, etc, as well as in some civil designs, such as the Boeing SST 2707, which was cancelled before being built.
However, on very rare occasions the concept described has been used for variable sweep for aircraft stabilising surfaces, such as in the Tupolev 144, where the horizontal stabilising surface is located in the front part of the fuselage, in front of the main aerofoils (canard-type configuration) and is also of variable geometry. In some aircraft configurations in which the sweep of the aerofoils is variable, such as those described in the documents U.S. Pat. No. 4,569,493 and U.S. Pat. No. 5,984,231, for example, the stabilising surfaces or control surfaces vary their sweep with the aerofoils, as the stabilising surfaces are arranged in the former, moving together with them. The problem with this configuration is that the control surfaces are less efficient aerodynamically, hence they need to be larger, which negatively affects aircraft weight. On the other hand, these control surfaces have geometric limitations imposed on them by the sweep angle of the aerofoils on which they are located.
According to the configuration of the aircraft described in document GB 664,058, which is considered to be the closest document to the above technique, this aircraft comprises aerofoils (wings) with variable sweep, as well as comprising tail stabilising surfaces also with variable sweep to increase the lift at the tail of the aforementioned aircraft, such that the sweep angle these tail stabilising surfaces adopt is in the same direction and of approximately the same magnitude as that adopted by the aerofoils or wings. With the above variable sweep configuration, the aircraft is capable of adapting to the necessary and conflicting requirements for flight at high and low speed. However, a configuration of this type raises problems inasmuch as each of the tail stabilising surfaces transmits bending moments to the connection point on the aircraft fuselage on which they are located. So, the fuselage needs to support very high bending moments, hence it needs to be designed in such a way as to be sufficiently resistant to support these moments, which makes the weight of the aircraft high, this being an especially undesirable characteristic for aircraft. Additionally, the mechanism which makes each tail stabilising surface rotate is very complex.
An alternative way of obtaining a variable sweep aerofoil configuration consists of making the complete wing pivot with respect to the fuselage around a vertical axis. This configuration is known as oblique wing and has only been employed experimentally in an aeroplane at full size in the case of the NASA Dryden D-1. The use of variable sweep horizontal stabilising surfaces with the pivoting configuration in the style of the oblique wing is unknown, however, in any aeroplane, including those mentioned previously with variable sweep wings.
From the description of the favourable and adverse effects of the sweep angle in the aerofoils and, in particular, in the stabilising surfaces, it is gathered that sweep angle is desirable for high-speed flight, but reduces the effectiveness of the stabilisers at low speed. Therefore, it would be desirable to be able to set the optimum sweep for the stabilisers for each phase of flight, according to aircraft flight speed, without incurring the complexity and weight associated with the known solutions for sweep variation mechanisms.
This invention is aimed at overcoming the aforementioned problems.